The present invention is directed generally to systems for displaying information, and more particularly to reflective projection systems.
Optical imaging systems typically include a transmissive or a reflective imager, also referred to as a light valve or light valve array, which imposes an image on a light beam. Transmissive light valves are typically translucent and allow light to pass through. Reflective light valves, on the other hand, reflect only selected portions of the input beam to form an image. Reflective light valves provide important advantages, as controlling circuitry may be placed behind the reflective surface and more advanced integrated circuit technology becomes available when the substrate materials are not limited by their opaqueness. New potentially inexpensive and compact liquid crystal display (LCD) projector configurations may become possible by the use of reflective liquid crystal microdisplays as the imager.
Many reflective LCD imagers rotate the polarization of incident light. In other words, polarized light is either reflected by the imager with its polarization state substantially unmodified for the darkest state, or with a degree of polarization rotation imparted to provide a desired grey scale. A 90xc2x0 rotation provides the brightest state in these systems. Accordingly, a polarized light beam is generally used as the input beam for reflective LCD imagers. A desirable compact arrangement includes a folded light path between a polarizing beamsplitter (PBS) and the imager, wherein the illuminating beam and the projected image reflected from the imager share the same physical space between the PBS and the imager. The PBS separates the incoming light from the polarization-rotated image light. A single imager may be used for forming a monochromatic image or a color image. Multiple imagers are typically used for forming a color image, where the illuminating light is split into multiple beams of different color. An image is imposed on each of the beams individually, which are then recombined to form a full color image.
It is desirable to use as much light generated by the light source as possible. Where the light source generates light over a wide angle, such as an arc lamp, more light can be passed through the imager system using high f-number optics. A problem, termed xe2x80x9cpolarization cascadexe2x80x9d and associated with a conventional PBS, places a lower limit on the f-number of the illumination optics of traditional optical imaging systems. A conventional PBS used in a projector system, sometimes referred to as a MacNeille polarizer, uses a stack of inorganic dielectric films placed at Brewster""s angle. Light having s-polarization is reflected, while light in the p-polarization state is transmitted through the polarizer. However, wide angle performance is difficult to achieve using these polarizers, since the Brewster angle condition for a pair of materials is strictly met at only one angle of incidence. As the angle of incidence deviates from Brewster""s angle, a spectrally non-uniform leak develops. This leak becomes especially severe as the angle of incidence on the film stack becomes more normal than Brewster""s angle. Furthermore, there are contrast disadvantages for a folded light path projector associated with the use of p- and s-polarization.
Since light in a projection system is generally projected as a cone, most of the rays of light are not perfectly incident on the polarizer at Brewster""s angle, resulting in depolarization of the light beam. The amount of depolarization increases as the system f-number decreases, and is magnified in subsequent reflections from color selective films, for example as might be found in a color-separating prism. It is recognized that the problem of depolarization cascade effectively limits the f-number of the projection system, thereby limiting the light throughput efficiency.
There remains the need for an optical imaging system that includes truly wide-angle, fast optical components that may allow viewing or display of high-contrast images with low optical aberration.
Generally, the present invention relates to an apparatus for reducing astigmatism in a projection system that is particularly well suited to reducing astigmatism in LCD projection systems. In particular, the invention is based around an imaging core that includes astigmatism reduction in at least one of its elements, for example in the polarization beamsplitter or, where the imaging core includes imagers for two or more color bands, in the color combiner such as a color prism, an x-cube combiner or a two-color dichroic combiner.
One particular embodiment of the invention is directed to an optical device that includes a polarizing beamsplitter, a first path being defined through the polarizing beamsplitter for light in a first polarization state, and at least one imager disposed to reflect light back to the polarizing beamsplitter, portions of light received by the at least one imager being polarization rotated, polarization rotated light propagating along a second path from the imager and through the polarizing beamsplitter. An astigmatism compensating element is disposed on the second path to reduce astigmatism in the polarization rotated light caused by the polarizing beamsplitter.
Another embodiment of the invention is directed to an optical device that includes polarizing beamsplitter means for directing light in a first polarization state along a first path and for directing light, in a second polarization state orthogonal to the first polarization state, along a second path different from the first path, and light imaging means for imposing an image on light by rotating polarization of portions of the light and reflecting the light to the polarizing beamsplitter, image light propagating along the second path through the polarizing beamsplitter means. The device also includes astigmatism correcting means disposed on the second path to reduce astigmatism in the image light caused by the polarizing beamsplitter means.
Another embodiment of the invention is directed to a projection system that includes a light source to generate light, conditioning optics to condition the light from the light source and an imaging core to impose on image on conditioned light from the conditioning optics to form image light. The imaging core includes a polarizing beamsplitter and at least one imager, and at least one element in the imaging core is adapted to reduce astigmatism in the image light. A projection lens system projects the astigmatism-reduced image light from the imaging core.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description, which follow, more particularly exemplify these embodiments.